Arches has one of the most dramatic park entrances I can think of anywhere. From the main highway and visitor center, the park road climbs steeply up a round amphitheater eroded from the footwall of a normal fault developed on one of the salt anticlines that traverse the region. In the picture above, the rocks on the right that are sloping downward to the left are Jurassic in age. The rocks of the cliff on the left are Triassic rocks, millions of years older. The fault has caused the Jurassic rocks to sink downwards against the older sequence.

Our first order of business was to revisit our sunset spot of the previous evening. I wanted pictures of the same scenes in the morning light. The "nativity scene" profile morphed into spires and towers spread across a long valley.

Comparing the pictures is almost like looking at negative images; instead of black rock against a blazing yellow-orange sky, there was blazing orange and red rock against a dark blue sky.

We continued up the road to the end at Devil's Garden. Along the way we got some nice perspectives of the sandstone fins.

The fins formed as the solid sandstone stretched and sank into the salt anticlines, breaking in parallel joints during the process. The fins are the starting point for the formation of arches.

The sand between the fins holds groundwater long enough to dissolve the chemicals holding the rock together. Small arches form at the base of the fins, and grow larger as sand and occasionally larger chunks fall from the interior face of the growing arch. A small arch can be seen at the base of the fin in the picture below. For some reason whenever I see this outcrop I am reminded of the Jawa sandcrawlers from the first Star Wars movie....

One of the most accessible arches is Skyline Arch, which forms a nice backdrop for the campfire theater at the campground in Devil's Garden. It takes a bit of scrambling to reach it, but it is well worth the effort.

The arch opening was once much smaller. A big chunk fell in 1940 or so, increasing the size of the arch by 40%. A little huffing and puffing and I was standing in the opening.

It is much steeper on the other side! There is a sheer drop-off, and a maze of fins off to the southwest. The scale is hard to perceive; click on the picture below and see if you can find the person standing on the rocks below the arch.

Skyline Arch has a special place in my memories. When I conducted my very first southwest geology trip with my students in 1989, we (all eight of us) had walked up to Skyline to see the sunset. As we sat there in the growing darkness, a full moon was rising over the La Sal Mountains. It was a stunning sight.

We made a short stop at the Fiery Furnace. I haven't had a chance to wander through this part of the park. Apparently it is an easy place to get lost...one needs a permit or a ranger guide to explore it!

We took one more long look at the La Sal Mountains, and worked our way out of the park and onto the highway. We had some miles to go...we were headed to Capitol Reef National Park. More in the next post!

Monday, November 28, 2011

Last summer we set out on a vagabonding journey that roughly followed the 39th Parallel across the western states of Nevada, Utah and Colorado. By vagabonding, I refer to the set of loose rules we followed to not plan more than a day or two in advance, and try to see as many new sights as we could see. We ultimately made it to Rocky Mountain National Park, and now after 10 days on the road, we were headed west, slowly returning home to California. Our winding road brought us to the town of Moab, Utah, and Arches National Park. We were now on the Colorado Plateau, a vast area of the earth's crust that was stable for hundreds of millions of years, but in recent geologic time has been uplifted and deeply eroded by the Colorado River and her tributaries.

I've been to Arches National Park a great many times, but almost always with a large group of students. A group necessitates some serious planning, and our trips usually include the same three places: a hike to Delicate Arch for the sunset, a hike to Double Arch, and a hike to Landscape Arch, the largest in the park (and probably in the world). On this particular day, our new experience was stopping and sitting at one of the less iconic viewpoints. Literally sitting, for a rather long time. We had a picnic dinner, and just experienced the desert while the sun set behind us.

Before settling in, we made a quick circuit past the La Sal Mountains overlook and the Windows Section. The La Sal Mountains are outside of Arches National Park, but provide a spectacular backdrop to arches and fins that make up most of the scenery in the park. The mountains have an unusual origin. They are not quite volcanic in origin (although volcanoes almost surely existed here, but have been eroded away), but instead are formed of intrusive rock that reached shallow depths in the crust, wedging and bulging up the sedimentary layers that lay above the molten rock. The intrusions are called laccoliths. The mountains rise to 12,000 feet, and are often snow-covered, even in early summer.

The Windows Section contains several large arches, including Double Arch, made famous by the opening scenes of Indiana Jones and the Last Crusade. There is no deep cave up in the alcove, and no Coronado's Cross, though. Just a spectacular double arch. It's big; click on the photo below and look for the people walking under the arch.

The Windows Section is a good place to learn the stratigraphy of Arches National Park (see the picture below). There are three important sedimentary layers that are fundamental to the scenery, all Jurassic in age. The Navajo Sandstone is the preserved remains of a vast dune sea (an erg) that developed along the western edge of the supercontinent Pangea. It makes up much of the spectacular scenery at other parks like Zion and Capitol Reef, but is less visible in Arches. It makes up the lighter-colored layer at the base of the slope in the picture below.

The layers atop the Navajo Sandstone are two members of the Entrada Sandstone: the Dewey Bridge member and the Slickrock member. The Dewey Bridge formed in a tidal flat environment and has variable layers of siltstone and sandstone. It is easily eroded, which allows it to undercut the harder Slickrock member, causing the cliffs and fins in which the arches are developed. It often has a wavy "crinkled" aspect that has led some geologists to speculate that the layer was affected by the nearby impact of an asteroid at Upheaval Dome. The speculation is controversial, but the explanation appeals to me simply on the scale of grandeur and in thinking outside the box.

The cliffs at Arches are made of the Slickrock member of the Entrada Sandstone. The formation preserves sand dunes that developed along the coast of Pangea (much like the Navajo Sandstone). The arches form most often in the Slickrock member. I wrote about the formation of the arches in my series Time Beyond Imagining. Here is a short excerpt:

"The origin of the park's famous arches (there are hundreds) is tied to the salts of the Paradox Basin. The salt beds are thousands of feet thick, and when placed under pressure, can slowly flow upwards through the overlying rock, forming domes and anticlines (upward pointing folds). Over millions of years, erosion removed thousands of feet of overlying sediment, and the salt was close enough to the surface to be affected by groundwater: it was dissolved away. The tops of the folds collapsed inwards, and the Entrada Sandstone was fractured (jointing) into a series of parallel fins. A large northwest trending anticline crosses the park; most of the arches occur along the flanks of the anticline.

The top of the fins are exposed to the arid desert climate, where the rock dries quickly following the infrequent storms. At the base of the fins, where the rock is buried by soil and loose sand, the rock may stay moist much longer. This leads to the solution of the cement holding the sandstone together, and the fins start to weather from below. Eventually a small window may open up, and falling slabs of rock enlarge the opening."

The sun was sinking lower towards the horizon. We sat in the silence and ate our sandwiches; a few cars stopped briefly and people stepped out to see what we were looking at, but they were mostly in a hurry to be someplace else.

Arches is an elemental place, as most desert landscapes are. There is sunlight, and there is rock. Everything else is secondary, the plants, the animals, the people. If you are caught in the open on a hot summer day, the elements are perilous. Survival is a crapshoot. In our insulated existence, the crapshoot becomes a source of great beauty, especially as the sun sinks and the searing heat dissipates. The rocks glow, and then slowly fade to darkness.

Our minds see meaning in the random shapes of the rocks, and we make stories of their origin. Some stories involve myths and heroes, while others involve jointing and frost wedging. I enjoy both kinds. What do you see here?

The fire in the sky died away, and stars emerged in the darkness. We gathered our things and drove back into town.

Friday, November 25, 2011

"Black Friday" depresses me. A religious holiday season introduced not by a ceremony like Advent or something, but millions of people worshiping the almighty dollar by donating many of them to corporations in exchange for cheap and increasingly shoddy merchandise. A day predicated on greed. Pure and simple avarice. Enough of preaching...I got as far away from town as I could. I headed up the hill to a spiritual place.

I bet no one has ever thought to take a picture of Half Dome from the top of Sentinel Bridge before!

Yosemite Valley in November is a place in transition. The fall colors have mostly faded, and only a dusting of snow has graced the high peaks. But there is such stunning beauty in this place.

Sentinel Rock (below) is the kind of spire that would be considered remarkable in any other setting; here in Yosemite it is sometimes overshadowed by other iconic rocks like El Capitan and Half Dome.

The Cathedral Rocks (below) are more often seen from the Bridalveil Falls side. From the east in El Capitan Meadow, the sun shines through the cleft known as the Gunsight.

Many people look at Bridalveil Falls and notice that one of the rocks seems to be close to falling over. The overhung cliff is called the Leaning Tower, for good obvious reasons.

No matter how many times I look at Half Dome, I never fail to be impressed by the scale of this incredible cliff. I take hundreds of pictures of it, but I can never resist taking just one more: here is today's shot.

When you get a chance to visit Yosemite Valley, first take it. Don't say "later"; you never know what will change in your life. Then, when you have seen all you can and you are leaving, stop at Valley View in the late afternoon or at sunset and just wait for magic to happen...

Thursday, November 24, 2011

...for the privilege of having a career that allows me to show students the beautiful places of the world, for digital cameras that allow me to preserve the memories, a forum to share my images with anyone that cares to explore them, and for all of you who take the time to read and comment. I really appreciate your visits to this blog. Please enjoy a random selection of my favorite images and places that have appeared in Geotripper at one time or another. Follow the links for the story behind the pictures.

About Me

I am a teacher of geology at Modesto Junior College and former president of the National Association of Geoscience Teachers, Far Western Section. I have led field trips all over the western United States, and a few excursions overseas, but my homebase is the Sierra Nevada, the Great Valley, and the Coast Ranges of California.

Blooks, Boogers and Bleries

Here are some of the series I've produced for the Geotripper Blog:

The Other California: what to see when you've seen all the really famous places in the Golden State (in progress). Into the Great Unknown: A rafting journey down the Colorado River through the Grand Canyon. The most incredible adventure of my life (so far)!